Effect of naturally occurring coumarins on the formation of epidermal DNA adducts and skin tumors induced by benzo[a]pyrene and 7,12- dimethylbenz[a]anthracene in SENCAR mice

Several naturally occurring coumarins previously found to be potentinhibitors of mouse hepatic ethoxyresorufin-O-deethylase (EROD) and/orpentoxyresorufin-O-dealkylase (PROD) were examined for their effects onformation of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene(DMBA) DNA adducts in mouse epidermis, as well as, their effects on skintumor initiation by these polycyclic aromatic hydrocarbons (PAH).Bergamottin, a potent inhibitor of hepatic EROD, given topically 5 minprior to an initiating dose of B[a]P, significantly decreased totalcovalent binding of B[a]P to DNA in a dose-dependent manner 24 h aftertreatment. A dose of 400 nmol bergamottin reduced covalent binding of B[a]Pby 72%. Coriandrin, at a dose of 400 nmol also significantly reduced totalcovalent binding of B[a]P by 59%. In addition, formation of the major(+)anti-B[a]P-diol epoxide-N2-dGuo adduct was selectively reduced by bothof these coumarins. In contrast, bergamottin and coriandrin did notsignificantly decrease covalent binding of DMBA to epidermal DNA at dosesof either 400 nmol or 800 nmol. Imperatorin and isopimpinellin, which aremore potent inhibitors of hepatic PROD activity, significantly reducedoverall binding of DMBA to epidermal DNA by 67% and 52%, respectively, whenapplied at doses of 400 nmol. These two coumarins also inhibited B[a]P-DNAadduct formation at similar doses but to a lesser extent. Imperatorin at adose of 400 nmol dramatically decreased formation of covalent DNA adductsderived from both the anti and syn diol epoxides of DMBA. Bergamottin was apotent inhibitor of tumor initiation by B[a]P while coriandrin was lesseffective in this regard. Imperatorin was an effective inhibitor of skintumor initiation by DMBA and also inhibited complete carcinogenesis by thisPAH. At dose levels higher than those effective against DMBA, imperatorinalso inhibited tumor initiation by B[a]P. The results demonstrate thatseveral naturally occurring coumarins possess the ability to block DNAadduct formation and tumor initiation by PAHs such as B[a]P and DMBA. Themechanism for reduced DNA adduct formation and tumor initiation appears toinvolve inhibition of the P450s involved in the metabolic activation ofthese hydrocarbons. Finally, the differential effects of certain coumarinson B[a]P vs DMBA DNA adduct formation and tumor initiation may be usefulfor dissecting the role of specific cytochromes P450 in their metabolicactivation.